Vapor condensation in insulated structures degrades their physical properties as well as thermal performance. Motion of the condensate within the insulation is a crucial factor in analyzing annual moisture retention in building shells. The diffusion of liquid can be caused by several driving potentials. A model for isothermal liquid transport in fibrous insulation is presented. The model relates liquid diffusion to the characteristics of the insulation. These are identified as void-fraction and its spatial distribution, mean fiber radius, directionally index, and tortuousity factor. Due to the anisotropic layered structure of the fiberglass insulation, liquid diffusion along the three principal directions are studied. The diffusion of liquid from one layer to the next is controlled by liquid diffusion along the layers and the tortuosity factor. Diffusion along the layers is caused by the suction potential and retarded by the viscous forces. By balancing the suction and the viscous forces, a model for transient diffusion in terms of the five medium properties is obtained. An electrical resistance probe for in-situ measurement of liquid content in insulation samples of high voidage has been developed. Data on liquid diffusion under isothermal concentration gradients for commercial fiberglass insulations have been acquired. The experimental data show consistent agreement with the model predictions.

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